Melittin inhibition and uncoupling of spinach thylakoids

Melittin has been found to inhibit a photosystem I reaction (diaminodurene to methylviologen) in much the same way that it inhibits sequential electron transport through both photosystems (water to methylviologen). At much lower concentrations melittin uncouples ATP synthesis. Melittin inhibition and uncoupling are found to be irreversible indicating very tight association between melittin and the membrane. Melittin inhibits the light-induced proton pump and the light-induced thylakoid Mg +2-ATPase activity as well as the Ca +2-ATPase activity of isolated coupling factor. The results are consistent with both a conventional model where the uncoupling by melittin is related to its lytic properties and a model wherein melittin interacts directly with coupling factor causing an uncoupling condition.